This is a trade token. It's used for getting goods from a certain person or company, but the coin could only be spent with that certain person or company. Here's what Wikipedia has to say about these tokens:
"Generally they have a merchant's name or initials, sometimes a town and state, and a value, such as "good for 5¢" (or other denomination) legend somewhere on the token."
I have no idea if the G on the coin is of a town, company, or person, though. Hope that helps a bit.
-Patrick
Edit: Further reading reveals that these were most commonly used from 1870 - 1920. Here's the link in case I missed anything:
http://en.wikipedia.org/wiki/Token_coin